pubs.acs.org/Langmuir © 2009 American Chemical Society
Nanostructure-Dependent Water-Droplet Adhesiveness Change in Superhydrophobic Anodic Aluminum Oxide Surfaces: From Highly Adhesive to Self-Cleanable )
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Wonbae Lee,†,# Bo Gi Park,‡,# Dae Hyun Kim, ,^ Dong Jun Ahn,§ Yongdoo Park, Sang Hoon Lee,†,‡, and Kyu Back Lee*,†,‡,
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† Department of Biomedical Engineering, College of Health Science, Korea University San 1, Jeongneung-3-dong, Seongbuk-gu, Seoul 136-703, Korea, ‡Department of Interdisciplinary Bio/Micro System Technology, College of Engineering, and §Department of Chemical and Biological Engineering, College of Engineering, Korea University, Seoul 136-701, Korea, Department of Biomedical Engineering, College of Medicine, Korea University 126-1, Anam-5-ga, Seongbuk-gu, Seoul 136-705, Korea, and ^ Analysis Development Team, Nong Shim Co., LTD, 370-1 Shindaebang-Dong, Dongjak-gu, Seoul 156-709, Korea. # These authors contributed equally.
Received August 21, 2009 Water-droplet adhesiveness was freely controlled on a single platform of superhydrophobic anodized aluminum oxide (AAO) within the range from highly adhesive to self-cleanable. Changing the structure from nanopore to nanopillar arrays at the surface caused a dramatic increase in the receding angle and a decrease in the hysteresis of water contact angles. The presence of dead-end nanopores but not through nanoholes was recognized as one of the main causes of the adhesiveness of superhydrophobic surfaces. The adhesiveness-controllable superhydrophobic AAO can be an excellent platform on which to elucidate the physical nature of the wetting phenomenon related to the nanostructure and has promising potential in technological applications.
Since the lotus leaf was recognized as an ideal model for selfcleaning surfaces,1 the mimetic fabrication of superhydrophobic surfaces has been an attractive goal in various fields of industry for the removal of undesirable contaminants from the surfaces of high-end products. Recently, adhesive superhydrophobic surfaces, which allow the placement of an almost completely spherical water droplet at a specific position such as a rose petal, have also garnered interest because of their promising potential in droplet-based technologies.2-5 The superhydrophobicity of a surface is generally defined by a static water contact angle (CA) greater than 150.6-8 Surface coverage with a low surface free energy material causes an increase of water CA, but the extreme CA value corresponding to superhydrophobicity cannot be achieved on a plain surface. An appropriate structure on the surface on the nanometer or micro*Corresponding author. Tel: þ82-2-940-2882. Fax: þ82-2-929-8044. E-mail:
[email protected].
(1) Blossey, R. Nat. Mater. 2003, 2, 301–306. (2) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Langmuir 2008, 24, 4114–4119. (3) Hong, X.; Gao, X.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478–1479. (4) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. Adv. Mater. 2005, 17, 1977–1981. (5) Winkleman, A.; Gotesman, G.; Yoffe, A.; Naaman, R. Nano Lett. 2008, 8, 1241–1245. (6) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626–5631. (7) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754–5760. (8) Zhang, H.; Lamb, R. N.; Cookson, D. J. Appl. Phys. Lett. 2007, 91, 254106-3. (9) Yang, C.; Tartaglino, U.; Persson, B. N. J. Phys. Rev. Lett. 2006, 97, 116103(4). (10) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193–202. (11) Manca, M.; Cortese, B.; Viola, I.; Arico, A. S.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 1833–1843. (12) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966–2967. (13) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235–3237. (14) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633–2637.
1412 DOI: 10.1021/la904095x
meter scale is required to achieve superhydrophobicity.9-15 The adhesiveness of a surface is closely correlated to the receding angle or to the hysteresis,16 which is defined as the difference between the advancing angle and receding angle:17 low adhesiveness is generally observed when hysteresis is minimized. In this study, a superhydrophobic aluminum surface, the water-droplet adhesiveness of which can be freely controlled within the range from highly adhesive to self-cleanable according to the nanostructure at the surface, is demonstrated. The discovery by Masuda and co-workers18 of self-ordering nanopores in anodized aluminum oxide (AAO) surfaces has prompted significant interest in the nanofabrication of various materials. The major experimental parameters that determine the structural features of the porous AAO structures have been previously reported with various self-ordering regimes.18-21 We prepared well-ordered AAOs through a two-step anodization by the wellknown self-ordering regime and modified the surface structure on the basis of the following principle. (See Supporting Information for the fabrication method in detail.) There is a peculiar wet etching property of nanoporous AAO. It is etched anisotropically under acidic wet etching conditions because the walls of the pores in AAO are more prone to etching (15) Pacifico, J.; Endo, K.; Morgan, S.; Mulvaney, P. Langmuir 2006, 22, 11072– 11076. (16) Schmidt, D. L.; Brady, R. F.; Lam, K.; Schmidt, D. C.; Chaudhury, M. K. Langmuir 2004, 20, 2830–2836. (17) Bistac, S.; Kunemann, P.; Schultz, J. J. Colloid Interface Sci. 1998, 201, 247– 249. (18) Masuda, H.; Fukuda, K. Science 1995, 268, 1466–1468. (19) Lee, W.; Ji, R.; Gosele, U.; Nielsch, K. Nat. Mater. 2006, 5, 741–747. (20) Nielsch, K.; Choi, J.; Schwirn, K.; Wehrspohn, R. B.; Gosele, U. Nano Lett. 2002, 2, 677–680. (21) Li, F.; Zhang, L.; Metzger, R. M. Chem. Mater. 1998, 10, 2470–2480. (22) Choi, D. H.; Lee, P. S.; Hwang, W.; Lee, K. H.; Park, H. C. Curr. Appl. Phys. 2006, e125–e129. (23) Jee, S. E.; Lee, P. S.; Yoon, B.-J.; Jeong, S.-H.; Lee, K.-H. Chem. Mater. 2005, 17, 4049–4052.
Published on Web 12/29/2009
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than are the upper surfaces.22,23 (In the remainder of this discussion, we will refer to the wet etching process after the fabrication of nanoporous AAO as “postetching”.) The packing density of AAO is known to be high in the central area of the borderlines (the dashed lines in Figure 1) between the pores and highest at the triple cross junction of the lines (the black dots in Figure 1). Thus, the locally dense regions are more resistant to acidic wet etching to form a sharp nanopillar array from the hexagonally packed nanopores. According to the structural changes on the surface of AAO during postetching, we divided the postetching period into four successive stages: (1) the initial stage (I stage, from 0 to 50 min of postetching), in which the widening of pores and the sharpening of superficial nanostructures at the triple junctions occur; (2) the maturation stage (M stage, from 50 to 80 min of postetching), in which the destruction of the pore walls progresses; (3) the pillar-formation stage (P stage, from 80 to 90 min of postetching), in which sharp nanopillars are formed; and (4) the collapse stage (C stage, after 100 min of postetching), in which nanopillars collapse because of excessive etching. Figure 2 shows typical SEM images of AAO surfaces at each stage. The important point is that the borderlines (the walls between the hexagonally packed nanopores) remained intact during the I stage and became sharper and the critical changes in the innate structure of AAO occurred during the M stage, from the nanopores to the nanometer-scale pillar array. After the formation of these appropriate nanostructures in each stage, a self-assembled monolayer (SAM) of heptadecafluoro-1,1,2,2-tetrahydrodecyl-trichlorosilane (HDFS, Gelest Inc.) was fabricated on the AAO surfaces. The advancing and receding CAs were obtained from more than four points in each sample by the sessile-drop method with four cycles using a DSA100 goniometer (KRUSS, Germany). The dynamic CAs of the HDFS-modified AAOs (HAAOs) are plotted against the duration of postetching in Figure 3. (See also Supporting Information Table 1.) The macroscopic observations in each stage are summarized as follows: (1) Conventional superhydrophobicity was achieved with the HAAOs after 50 min of postetching (HAAO50), but the hysteresis was still large. (2) The receding CA increased dramatically during the M stage, which resulted in a decrease in hysteresis. (3) Hysteresis-minimized or self-cleanable superhydrophobicity, which causes water to be highly repelled such that a water droplet is difficult to position on a surface, was achieved in the P stage on HAAOs after 80 and 90 min of postetching (HAAO80 and HAAO90). It is interesting that the increase in the receding angle or the decrease in hysteresis was closely related to the change in nanostructures on the surface from the nanopores to the nanopillar arrays. The superhydrophobicity of a rough surface has been understood through several theoretical models.24-28 The Wenzel and Cassie-Baxter models constitute two contrasting extremes. The Wenzel model assumes that the void space of the pores is completely filled with water, and the Cassie-Baxter model emphasizes the role of the water-repellent air cushion in the intaglio part of the surface.24-26,29 An adhesive hydrophobic (24) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (25) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (26) Wang, S.; Jiang, L. Adv. Mater. 2007, 19, 3423–3424. (27) McHale, G. Langmuir 2007, 23, 8200–8205. (28) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097–2103. (29) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457–460.
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Figure 1. Schematic top view of the hexagonally packed AAO.
surface with large hysteresis has been generally explained by the Wenzel model29 because the hydrophobic adhesiveness is currently understood by means of the large contact area of a water droplet on the solid surface, even if the surface is covered with materials of low surface free energy.2,29 Recently, a Cassie impregnating wetting state was also suggested in order to understand the high adhesive force of a hierarchical structure such as a rose petal, which has both wetted grooves (Wenzel model) on the micrometer scale and dried solid plateaus (Cassie model) on the nanometer scale.2 Our adhesive superhydrophobic surfaces during the M stage did not match the descriptions provided by the Wenzel or the Cassie impregnating state. The surfaces have just the specific nanopore structures without hierarchy, and the calculated static CA based on Wenzel’s model was unphysical in that it was far higher than the angle limit of 180, even for an unetched nanoporous HAAO surface because of the high aspect ratio of the densely packed nanopores (Supporting Information). It is reasonable to adopt an intermediate30 or transitional26 state between the Wenzel and Cassie-Baxter models, which describes the situation in which a water droplet is partially withdrawn into the nanopores and the captured air exists in the pores. The receding angle remained consistently low (
